DETERMINING THE ORIGIN OF THE MONOCEROS RING Chloe Keeling University of Tennessee HEP Seminar April 19 2017 OUTLINE

• Morphology of the Milky Way • Explanations for Monoceros Tidal Stream • Tidal Stream • Flared Thick Disk • Disrupted Disk • Conclusion GALACTIC COORDINATES Galactic Longitude: l Galactic Latittude: b

Auger.org STRUCTURE OF THE MILKY WAY Velocity Dispersions: Thin: 15-25 km/s Thick: ~45 km/s Halo: ~100 km/s

Swinburne University

BASIC PROPERTIES OF THE MONOCEROS RING

• Over density of stars in the region of 100° < < 230° and 30° < < 35° • ~25 kpc from galactic center 𝑙𝑙 − 𝑏𝑏 • [Fe/H]= 0.95 ± 0.15 • Co-rotating with the disk • Scale Height: ~3.5 kpc • Velocity Dispersion: ~25 km/s • Stellar Mass in all of Monoceros: 107-108 M 7 • Observable Stellar Mass : 3x10 M ⊙

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POSSIBLE EXPLANATIONS FOR THE MONOCEROS RING

• 1. Tidal Stream • 2. Flared and warped thick disk • 3. Disrupted thin disk (north/south stellar density asymmetry) EVIDENCE FOR TIDAL OR HALO ORIGIN

• Presence of RR Lyrae variable stars (Mateu et al. 2012) • Globular clusters with orbits that match Monoceros (Frenchaboy 2006) • Non-zero radial velocity at l=180 • Canis Major: Dwarf Galaxy located∘ at (l,b)=(240 , -8 ) • Model by Peñarrubia et al. 2005 produced correct∘ b ∘values and velocities with 10-20 km/s of observation • We aren’t sure this is actually a dwarf galaxy is could be an outer spiral arm that is observable at the (l, b) values because of the galactic warp

HR DIAGRAM EVIDENCE CONTRARY TO TIDAL OR HALO ORIGIN

• Presence of M giants (Indicative of Metal Rich origin)

• Present at low galactic latitude

• Nearly circular prograde orbit (models of tidal streaming events have had significant difficulty matching)

• Same model by Peñarrubia et al. 2005 has problems filling in the band like structure observed FLARED DISK EVIDENCE FOR A FLARED THICK DISK

• Present at low galactic latitude • Nearly circular prograde orbit • Presence of M giants (Indicative of Metal Rich origin) • Canis Major can be explained by a warped disk + spiral arm

EVIDENCE CONTRARY TO FLARED THICK DISK

• Evidence for small scale height of the thick disk ~1.8 kpc (Cheng et al. 2012)

• Presence of RR Lyrae variable stars

• North-South stellar density assymetry DISRUPTED DISK HESS DIAGRAM WHAT DID I DO ABOUT IT? Metallic Line A type Stars: Chemically peculiar spectral lines This is caused by gravitational settling and radiative levitation of the various elements in the quiescent atmosphere of non- convective, slowly rotating A- stars

OBJECTIVE

• Determine the frequency with which A type stars present with peculiar spectral lines in globular clusters, in the thick disk, and in Monoceros Ring

• Characterize the A type stars • Determine proper motions • [Fe/H] • [α/H]

SAMPLE OF FIELD STARS

• 5865 blue stragglers and 670 blue stragglers displaying the Am effect • bias to stars that are more metal rich and have high signal-to-noise ratios

• Determine Metallicty • -1.1<[Fe/H]<-0.5 • Found using kinematics and known iron deficiency of other Am stars

WHERE TO NOW?

• Further study of relevant globular clusters and halo stars.

• Further study and characterization of the Monoceros ring stellar population

• Further study and characterization of the thin and thick disk populations near Monoceros REFERENCES

• “Tidal Streams in the Local Group and Beyond: Observations and Implications” by Heidi Jo Newberg and Jeffery L Carlin (2015)